The strip that does the most work

Walk along any small brook in late June and the difference is obvious within a few paces. Where the bank has been left rough, the water runs cool and clear under overhanging leaves, the soil holds firm, and there is a low hum of insects in the herbage. Where the bank has been flailed to the edge, the same brook looks thin and bright, its water warmer, its soil bleeding into the channel with every spate, and very little living in it.

The margin — the rough ribbon of ground reaching roughly a metre or two either side of the water's edge — is the most productive strip of habitat on a small watercourse. It shades the channel, filters what runs off the land, holds the bank together with roots, and feeds insects that in turn feed the fish, birds and bats upstream and down. Planting it with natives is one of the quietest, cheapest and most satisfying improvements you can make to a brook you care about.

Native plants worth their place

Choose plants that genuinely belong to your stretch of country, and match them to how wet the ground stays. A few reliable stalwarts for British lowland brooks:

• Meadowsweet — tall, frothy cream flowerheads in midsummer, sweetly scented, happiest on damp mid-bank ground. Excellent for hoverflies and small beetles.
• Water mint — creeping, aromatic, with pale lilac whorls from July. Superb for bees and butterflies, and tolerant of ground that is wet almost year-round.
• Purple loosestrife — a magnetic plant for bees and day-flying moths, though keep it in ones and twos rather than drifts so it does not crowd out smaller species.
• Ragged robin — pink and untidy in the best way, flowering in damp meadow beside the bank, and a favourite of bumblebees.
• Yellow flag iris — bold, upright, useful for binding the soil at the water's edge. Lift and divide it every few years so it does not take over.
• Marsh marigold — early gold in March and April, when very little else is offering nectar, and perfectly happy with its feet in the wet.
• Sedges and soft rush — not glamorous, but the backbone of a stable margin. Their dense roots knit the bank far better than any flower.
• Alder and goat willow — for the upper bank, where their roots reach deep and their leaves dapple the water in summer.

Ask for British-grown stock, ideally grown from seed collected locally. Avoid named garden cultivars and anything double-flowered — they offer far less to wildlife.

Read your bank before you dig

Banks are not uniform, and plants that sulk in one place will romp away ten feet along. Take a slow walk and note where the ground squelches, where it dries out, and where the current scours in winter.

• The wet toe, at the water's edge: marsh marigold, water mint, brooklime, sedges.
• The mid-bank, damp but rarely submerged: meadowsweet, yellow flag iris, ragged robin, purple loosestrife.
• The upper bank and bank top, drier and often shaded: bird's-foot trefoil, guelder rose, hawthorn, alder, goat willow.

Leave the channel itself alone. Gravel, riffles and clean stones are habitat in their own right, and disturbing them does far more harm than good.

Planting that works with the water

Bare-root trees, shrubs and plug plants settle best between November and March, while the ground is damp and the plants are dormant. Space plugs roughly 30 to 45 centimetres apart so they knit together within two seasons but do not compete immediately. Water them in well, then leave them be.

Keep to a few simple rules. Use peat-free compost, and add no fertiliser at all — nutrients are the last thing a brook needs, and rich growth makes weak, floppy stems. On a bank that scours badly, pin coir or jute matting around the plugs until the roots take hold. Protect young trees with a stake and a guard, and if livestock graze the far side, fence a generous buffer rather than a token strip.

Do check what permissions apply before you start. In England, work in or near a main river generally needs consent from the Environment Agency, and work on an ordinary watercourse from the lead local flood authority; similar arrangements apply in Wales, Scotland and Northern Ireland. It is a short conversation that saves a great deal of trouble.

What the planting does for the water

Shade is the first gift. A margin that leans over the channel keeps summer water several degrees cooler, which matters enormously to invertebrates and small fish. The second is filtration: stems and roots slow surface runoff, trapping silt and taking up some of the nutrients that would otherwise feed algal blooms downstream. The third is structure — an architecture of roots, tussocks and dead stems that shelters everything from caddis larvae to water voles and overwintering insects.

Then there is the sheer abundance of life in the air above it. Meadowsweet, mint and loosestrife carry a summer-long supply of nectar and pollen, and the seedheads that follow feed finches and mice well into winter.

Slow management, slow living

A margin does not need much. Leave the seedheads standing through winter, then cut the herbage back in late summer or early autumn on a rotation, taking no more than a third of the length in any one year so there is always somewhere for insects to sit out the cold. Rake off the cuttings; leaving them smothers the plants underneath.

Deal with invasive non-natives early. Himalayan balsam should be pulled by hand before it sets seed, and Japanese knotweed needs specialist treatment rather than enthusiasm. Beyond that, the best management is patience: sit on the bank in July, watch what arrives, and let the brook do the rest.